Quinacridone, also referred to as 5,12-dihydroquino[2,3-b]acridine-7,14-dione, of the following formula ##STR1## is known to exist in three polymorphic modifications. The alpha (U.S. Pat. No. 2,844,484) and gamma (U.S. Pat. Nos. 2,844,581 and 2,969,366) forms are bluish-red pigments, while the beta form (U.S. Pat. No. 2,844,485) is a violet pigment.
A second, distinct beta quinacridone polytype designated .beta..sub.I was identified and described in U.S. Pat. No. 4,857,646. The new form differs from the known beta form by a modified X-ray diffraction pattern and a significant color shift from violet to magenta. The new form also offers a variety of new color styling opportunities, particularly for automotive and industrial finishes. The pigment additionally shows excellent hiding power, purity of shade, fastness to light and weathering, resistance to high temperatures and solvent and softener resistance.
Several methods were identified for preparing the new pigment and for preparing it in the desired relatively low specific surface area form. The new pigment could be prepared either by controlled particle size reduction/ripening of a crude pigment or by overmilling of a crude and then growing the pigment particles to the desired size and surface area. It was also noted that both particle size reduction and growth by crystallization could be achieved in a single operation.
Applicable disclosed procedures can be summarized as follows:
(1) Milling of alpha quinacridone, preferably of small particle size, in the presence of alcohol and base.
(2) Milling of crude or large particle size .beta..sub.II quinacridone (surface area of 3-8 m.sup.2 /g) in the presence of alcohol and base and appropriate milling media.
(3) Conducting the second procedure in two steps with premilling of the crude beta quinacridone following by ripening or particle size growth in a subsequent alcohol-base milling step.
(4) Particle ripening of a finished, particle size reduced beta quinacridone (a) by milling in the presence of alcohol and base, or (b) refluxing in basified alcohol.
Further experimentation with the new beta polytype revealed that it could exhibit a greater breadth of color within the magenta class. Thus, in terms of C.I.E. coordinates with L, A, B color space units, the modified form disclosed in U.S. Pat. No. 4,857,646 exhibited masstone L (lightness)=35.2 units, A (chroma)=38.0 units, B (hue)=10.9 units when incorporated into a high solids enamel paint system. It has now been determined that each of the L, A, B coordinates can be defined in terms of a range of values.